Many firearms, such as rifles, are equipped with an optical aiming device, such as a rifle scope, that provides the user with an image of an aligned aiming point or pattern (commonly known as a reticle) superimposed at the same focus as the target.
When shooting at long distances, shooters must adjust their aim to take into account the downward acceleration on the projectile imparted by gravity, which is often referred to as “bullet drop.” This is typically done by adjusting the angular position of the rifle scope relative to the rifle barrel using an elevation turret. A shooter may also adjust for left-to-right movement due to wind using a windage turret.
A zero point (also sometimes referred to as a “zero mark” or “zero location” or similar) for a rifle scope is determined when “sighting” a rifle, or other firearm, at a known distance by adjusting the angular position of the riflescope relative to the rifle barrel, via the elevation turret, until the impact point of the bullet matches the point on the target coincident with the optical center of the riflescope reticle. The zero point of the riflescope is set such that the rifle is sighted in at a first known distance, sometimes referred to as the “zero distance” or in some cases, the “zero-stop distance.” This known distance is typically the shortest distance for which the rifle is sighted. For targets at greater distances than the zero distance, the elevation turret is rotated to adjust the angular position of the scope with respect to the rifle barrel to compensate for the greater amount of bullet drop over the longer distance. For example, a zero-point of a turret may be set to correspond to a zero distance of 100 yards. For distances greater than the zero distance of 100 yards, a user rotates or “dials” the elevation turret a predetermined amount, often measured in minutes of angle (MOA) or mils, such that the riflescope is sighted in for the greater distance. For example, in a typical riflescope turret having sighting indicia, the indicium/number “0” may correspond to the zero-point distance of 100 yards; the number 3, or 3 MOA may correspond to 200 yards, and so on. When the “0” indicium on the turret is aligned with an alignment mark (“zero mark”) on the riflescope body, the rifle scope is sighted in (“zeroed in”) at the zero distance, e.g., 100 yards, such that a bullet should accurately strike a target located 100 yards from the user.
Known rifle scopes having adjustable turrets may also incorporate a “zero-stop” mechanism that serves to limit the rotation of the turret, and provides a mechanical indication of the zero point. For example, a turret having a zero-stop mechanism may limit rotation of the turret in one direction or another. Once a riflescope has been properly sighted, the zero point determined, and a zero-stop mechanism adjusted, the particular rotational position of the turret at which the turret is prevented from rotating will correspond to the zero point.